In recent years, the application range of magnetic recording apparatuses such as magnetic disk apparatuses, flexible disk apparatuses, and magnetic tape apparatuses has remarkably increased. As the importance of the above-mentioned magnetic recording apparatuses has increased, the recording density of magnetic recording media for use in these apparatuses has also been remarkably improved. In particular, since MR (Magneto-Resistive) heads and PRML (Partial Response Maximum Likelihood) signal processing techniques were introduced, the surface recording density has been dramatically increased. In recent years, GMR (Giant Magneto Resistive) heads, TMR (Tunneling Magneto Resistive) heads, and the like have also been introduced, and under such circumstances, the recording density has increased at a rate of as much as about 100% per year. The above-mentioned magnetic recording media have been required to have higher recording density, and therefore, magnetic recording layers have been required to have high coercive force, high signal to noise ratio (SNR), and high resolution. In recent years, efforts have also been continued to increase the surface recording density by increasing both the linear recording density and the track density.
The track density of the latest in magnetic recording apparatuses reaches as high as 110 kTPI (tracks per inch). However, the increase in track density is more likely to cause a problem in which pieces of magnetically recorded information interfere with each other between adjacent tracks so that the magnetization transition region at the boundary region becomes a noise source to degrade the SNR. This directly leads to an increase in bit error rate (BER) and therefore becomes a barrier to the improvement of recording density.
To increase the surface recording density, it is necessary to make finer the size of each recording bit on a magnetic recording medium and to make as great as possible the saturation magnetization and magnetic thickness of each recording bit. However, making a recording bit finer causes a problem in which the minimum magnetization volume per bit becomes so small that the recorded data may be lost by thermal fluctuation-induced magnetization inversion.
In addition, since the distance between tracks is reduced, very high accuracy track servo technology is required of magnetic recording apparatuses, and at the same time, the effect between adjacent tracks should be eliminated as much as possible during reproduction. In a method generally used, therefore, recording is performed in a wide area, and reproduction is performed in an area narrower than that for recording. This method can reduce the effect between tracks to the minimum but has a problem in which sufficient reproduction output is difficult to obtain, and therefore sufficient SNR is difficult to ensure.
A method being attempted to solve the problem of thermal fluctuation or ensure high SNR or sufficient output includes forming recesses and protrusions along tracks on the surface of a recording medium to physically separate the recording tracks from one another so that the track density can be increased. Hereinafter, said technique is referred to as a discrete track method, and the magnetic recording medium manufactured by the method is referred to as a discrete track medium.
A known example of the discrete track medium is a magnetic recording medium manufactured by a process including forming a magnetic recording medium on a nonmagnetic substrate having recess and projection patterns on its surface to form a magnetic recording track and a servo signal pattern physically separated from one another.
For example, the magnetic recording medium disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2004-164692 (hereinafter referred to as Patent Document 1) includes a substrate having recesses and protrusions on its surface, a ferromagnetic layer formed on the surface of the substrate with a soft magnetic layer interposed therebetween, and a protective film formed on the surface of the ferromagnetic layer. In this magnetic recording medium, a magnetic recording region that is physically separated from the surroundings is formed at the projection region.
Patent Document 1 discloses that the magnetic recording medium can inhibit the formation of magnetic domain walls in the soft magnetic layer, so that the effect of thermal fluctuation is less likely to be produced and that the interference between the respective recording bits can be prevented, which is effective in reducing record loss or noise caused by the interference between adjacent bits and therefore makes it possible to form a low-noise, high-density, magnetic recording medium.
For example, as disclosed in JP-A No. 2004-178793 (hereinafter referred to as Patent Document 2) and JP-A No. 2004-178794 (hereinafter referred to as Patent Document 3), discrete track methods include a method including forming a magnetic recording medium having several layers of thin films and then forming tracks, and a method including previously forming recess and projection patters directly on the surface of a substrate or on a thin film layer for forming tracks and then forming a thin film of a magnetic recording medium.
Among these methods, the method disclosed in Patent Document 2 is often called a magnetic layer processing type. Since in this method, physical processing is performed on the surface after the medium is formed, this method has the disadvantage that the medium can be easily contaminated during the manufacturing process, and the manufacturing process is very complicated. The latter method is often called an embossing type, in which the medium is hardly contaminated during the manufacturing process, but the recess and projection pattern formed on the substrate is transferred to the film formed, so that a problem occurs in the way that the recording/reproducing head which floats to the medium to perform recording and reproducing may have an unstable floating position or height.
JP-A No. 5-205257 (Patent Document 4), JP-A No. 2006-209952 (Patent Document 5), and JP-A No. 2006-309841. (Patent Document 6) disclose a process that includes forming an inter-recording-track region for a discrete track medium by the step of implanting ions of nitrogen, oxygen or the like into a preformed recording layer (magnetic layer) or applying laser beams to the preformed recording layer to change the magnetic characteristics of the part corresponding to the preformed recording layer, so that the recording track is formed.
In this method, however, the recording layer may be damaged by the ion implantation or laser irradiation, so that recesses and protrusions may be formed on the surface of the recording layer. This method also has a problem in which although the ions being implanted or the laser beams have high energy, the energy density per whole surface of the medium is low, so that a long treatment time is required to change the magnetic characteristics of the whole surface of the medium.
JP-A No. 2002-3591.38 (hereinafter referred to as Patent Document 7) discloses a method for patterning a magnetic material, which includes exposing an exposed part of the surface of a ferromagnetic layer of a magnetic recording medium to halogen-containing reactive gas to fluorinate the ferromagnetic material, so that the ferromagnetic material is converted into a non-ferromagnetic material.
The method disclosed in Patent Document 7 does not involve physical processing of the recording layer and therefore can reduce the contamination during processing of the recording layer. The method also makes it possible to modify the magnetic characteristics of the recording layer in a short time as compared with the case where ion beams or the like are used to modify the magnetic characteristics of the recording layer.
However, it has been found that when the method disclosed in Patent Document 7 is used, a fluoride such as cobalt fluoride is formed around the magnetic recording region and the recording layer of the magnetic recording region is gradually eroded by the fluoride. It has been found that particularly when a material capable of easily forming a compound with reactive plasma is used in a mask layer, the compound can easily diffuse from the mask layer to a magnetic recording region. It has also be found that when a hard disk drive produced with such a magnetic recording medium is used under a high-temperature, high-humidity environment, the magnetic recording/reproducing characteristics degrade over time.
The present invention has been made under the circumstances described above, in which a mask layer stable against reactive plasma is used to prevent it from forming a compound with halogen or the like. An object of the present invention is to provide a method for manufacturing a magnetic recording medium, which can prevent diffusion of a compound from the mask layer by the feature mentioned above and can produce a magnetic recording medium that has high magnetic recording pattern separation performance, is not influenced by signal interference between adjacent patterns, and has magnetic recording/reproducing characteristics improved to be less degraded over time even when it is used under a high-temperature, high-humidity environment and to provide such a magnetic recording medium and a magnetic recording and reproducing apparatus.